The present invention relates generally to voltage regulation in distributed power systems and, more particularly, to an AC voltage regulator module suitable for converting AC distribution bus voltage to low DC voltage.
An AC voltage regulator module (VRM) is an on-board power module in an AC distributed power system such as for computers, servers, and other electronic systems. An AC VRM performs two functions: (1) rectification, i.e., conversion of high-frequency AC bus voltage to DC output voltage, and (2) regulation, i.e., tight control of the amount of energy transferred to the load so that the output voltage is regulated regardless of load conditions.
Series resonant type circuits have been proposed as AC VRM's. In particular, regulated resonant rectifiers use a center-tapped transformer followed by a pair of rectifiers, e.g., synchronous rectifiers. Output regulation is achieved by using a variable resonant inductor, or a variable resonant capacitor (e.g., implemented by a switched capacitor), or phase angle controls (e.g., by using a shunt switch across the transformer primary or secondary side). Disadvantages resulting from separation of the rectification and regulation functions include complicated control, too many semiconductor devices, and relatively low efficiency.
Another approach is a primary-side controlled AC VRM wherein an AC switch on the primary side regulates the output voltage by controlling its conduction time. Synchronous rectification is provided by separate FET switches on the secondary side. Shortcomings include relatively high conduction losses due to the voltage drops across both the AC switch on the primary side and the synchronous rectifier on the secondary side, thus resulting in low efficiency.
Design issues for AC VRM's are high-efficiency, high current slew rate, fast control loop and simple topology. In particular, the speed of microprocessors and integrated circuits (IC's) continues to increase, accompanied by a decrease in supply voltage level. For example, DC supply voltage levels of 2.2V, 1.8V, and even voltages below 1V are in demand for next-generation microprocessors and IC's. The reduced output voltage level imposes great challenges on efficiency due in large part to the voltage drop in the rectification stage.
Accordingly, it is desirable to provide a high-efficiency AC VRM, even at low output voltage, high current and high switching frequency. It is also desirable to provide an AC VRM that combines the functions of rectification and regulation into one power conversion stage, thereby reducing conduction losses. It is furthermore desirable to provide an AC VRM that provides soft-switching operation to reduce switching losses.